The present invention relates to a ball bearing, and particularly a ball bearing used to support a rotary shaft, which is disposed in a fan motor of an electric cleaner for domestic use or in a blower of an air conditioner for domestic use and is to be rotated at a high speed with a low load, in such a manner that the rotary shaft can be rotated freely with respect to a housing.
Conventionally, such a ball bearing 1 as shown in FIG. 11 is widely used to support a rotary shaft, which is disposed in various apparatus, in such a manner that it can be freely rotated with respect to a housing. The ball bearing 1 comprises an outer ring 3 including on the inner peripheral surface thereof a deep-groove type of outer ring raceway 2 having an arc-shaped section, an inner ring 5 including on the outer peripheral surface thereof an inner ring raceway 4 having an arc-shaped section, and a plurality of balls 6 respectively interposed between the outer and inner ring raceways 2 and 4 so as to be free to roll;  and, the outer ring 3, inner ring 5 and balls 6 are all made of bearing steel such as SUJ2 or M50, ceramic, or the like. The balls 6 are respectively held by a retainer 7 in such a manner that they are able to roll while they are spaced from one another. Also, to the inner peripheral surfaces of the two end portions of the outer ring 3, there are secured the outer peripheral edge portions of sealed rings 8 and 8, whereas the inner peripheral edge portions of the sealed rings 8 and 8 are respectively disposed so as to be close and opposed to the outer peripheral surfaces of the two end portions of the inner ring 5.
By the way, in the case of the conventional ball bearing 1, generally, where the diameter of the respective balls 6 is expressed as Db, the radius of curvature of the section shape of the outer ring raceway 2 is expressed as Ro′, and the radius of curvature of the section shape of the inner ring raceway 4 is expressed as Ri′, the following equations are established; that is, 0.50<Ro′/Db≦0.53, and 0.50<Ri′/Db≦0.52. Also, where the outside diameter of the outer ring 3 is expressed as D, the inside diameter of the inner ring 5 is expressed as d, and the pitch circle diameter (P.C.D.) of the respective balls 6 is expressed as Dp′, the following equation is established; that is, Dp′≈(D+d)/2. In other words, there is employed the equation, that is, Dp′/(D+d)/2≈1, and the respective balls 6 are positioned substantially in the middle of the outer peripheral surface of the outer ring 3 and the inner peripheral surface of the inner ring 5 with respect to the diameter direction of the ball bearing 1.
In case where the above-structured ball bearing 1 is used to support, for example, the rotary shaft of a fan motor disposed in a suction device employed in an electric cleaner, the outer ring 3 is inserted and fixed to a fixed housing, while the inner ring 5 is outserted and fixed to the rotary shaft.
The above-mentioned conventional ball bearing 1 has a general-purpose structure which aims for assembly into one of various rotation support portions, but does not prefer to apply under the low-load and high-speed rotation condition, and, therefore, the rotation torque (rotation resistance) thereof is not always low. On the other hand, there has been increasing a demand for reducing the rotary torque of the rotation support portion in order to be able to cope with a rising energy saving tendency in recent years. In view of such circumstances, it is an urgent need to realize a ball bearing which not only provides a small rotation torque but also can be incorporated into the rotation support portion which rotates at a high speed with a low load. As the simplest means for reducing the rotation torque, it can be expected that, as grease to be applied to the portion where the balls 6 are disposed, grease having low viscosity is used. However, there is a limit to the torque reduction that can be realized by reducing the viscosity of the grease and, therefore, in order to be able to realize large torque reduction, it is necessary to change the structure of the ball bearing itself.
In case where the rotation torque of the rotation support portion rotating at a high speed with a low load is reduced by changing the specifications of the ball bearing, use of a ball bearing whose diameter and diameter-associated elements are reduced in size (that is, a small-sized ball bearing) can realize rather large torque reduction. However, in this case, it is necessary to reduce the inside diameter of a housing into which the outer ring is inserted and fixed, which unfavorably requires the design change of the remaining component members of the rotation support portion. Also, even in case where the diameter and its associated elements of the ball bearing are simply reduced in size, there still remains a possibility that sufficient torque reduction cannot be realized.